Many businesses use radio-frequency identification (RFID) tags to automatically identify and track objects such as packages, crates, pallets, and various products or other forms of inventory. These tags are generally placed on the objects using an automatic tag applicator, and then the objects are identified and tracked by one or more tag readers each including an antenna and an accompanying logic apparatus.
When an object having an RFID tag is placed near a tag reader, the RFID tag receives electromagnetic signals from the reader. In response to these electromagnetic signals, the RFID tag generates radio waves with a unique pattern identifying the RFID tag. The reader's antenna receives these radio waves and transmits them to the logic apparatus, which interprets the radio waves to determine the identity of the object. Examples of the general structure and operation of RFID tags are presented in a paper entitled “Antenna Design for UHF RFID Tags: A Review and a Practical Application”, IEEE Transactions on Antennas and Propagation, Vol. 53, No. 12, December 2005.
Ideally, a tag reader is able to quickly and reliably identify objects based on their RFID tags. For instance, where a tag reader tracks and identifies objects passing along a conveyor belt, the tag reader should be able to reliably identify the objects at a rate commensurate with the speed of the conveyor belt. If the tag reader station is too slow or unreliable, it may lose track of objects passing along the conveyor belt, or it may require the conveyor belt to pause or slow down, reducing the conveyor belt's efficiency.
One significant cause of failures in conventional tag readers is electromagnetic interference from objects being tracked. As an example, suppose an RFID tag is used to track a box containing electronic equipment such as a television. If the RFID tag is placed too close to electromagnetic components of the television, the television may interfere with the tag's reception of signals from a tag reader, or with the tag's transmission of radio waves to the tag reader. Because of this electromagnetic interference, the tag reader may fail to identify the RFID tag.
To illustrate this type of electromagnetic interference, FIG. 1 shows a box 105 including a large object 110 generating electromagnetic interference. For explanation purposes, we will assume that object 110 comprises a large metal cylinder. Two RFID tags are placed on box 105: one at a location “A”, and another at a location “B”. Because the tag at location “B” is closer to object 110 compared with the tag at location “A”, the tag at location “B” will experience relatively greater electromagnetic interference when communicating with a reader antenna 115. Accordingly, in the relatively simple example of FIG. 1, a tag reader can more reliably identify box 105 when the RFID tag is placed at location “A”.
In the relatively simple example of FIG. 1, an observer can intuitively recognize the advantage of placing an RFID tag at location “A” rather than at location “B” based on the shape and location of object 110. However, in boxes containing more complex objects, or objects arranged in more complex patterns, it becomes difficult for an observer to determine good locations for an RFID tag. For instance, in electronic equipment including complex parts made of metal, ceramics, and plastics, it may be difficult to determine which part of the equipment generates the most electromagnetic interference for an RFID tag. Moreover, to efficiently determine a relatively good location on for an RFID tag on a box, it may be desirable to avoid the need for an observer to inspect the box's contents.
To more efficiently determine appropriate locations for RFID tags, researchers have developed a number of techniques for automatically measuring patterns of electromagnetic interference in potential tag locations. These patterns are then relied on to place tags in locations with relatively low interference.
In some of these techniques, a machine holds an RFID tag and moves the tag to different locations on a box. At each location, the machine measures signal characteristics at both the RFID tag and a tag reader to determine whether or not the location allows reliable communication between the RFID tag and the tag reader. An example of one such technique is disclosed in U.S. Pat. No. 7,132,948 to Sweeney (Sweeney).
Similarly, in other conventional techniques, a machine moves a box to different locations relative to an RFID tag. As the box is moved to these different locations, the machine tests the reader's ability to communicate with the RFID tag. Based on these tests, the machine determines preferable locations for placing RFID tags. An example of one such technique is disclosed in a paper entitled “A System to Test the Performance of RFID-Tagged Objects”, by Hugo Mallinson et al, Proceedings of the 2007 International Symposium on Applications and the Internet Workshops (Mallinson).
Unfortunately, these conventional automated techniques have several shortcomings. One notable shortcoming is that these techniques tend to be slow. For instance, it may take a long time to move around a single tag or box as in Sweeney or Mallinson. As a result, these techniques take a long time to determine good locations for placing an RFID tag on a box.
Another shortcoming is that these techniques are not readily implemented in real environments where RFID tag reading generally occurs. For instance, machines for moving a box or tag around to different locations are not readily implemented along a conveyor belt system. Because these techniques are not readily implemented in real environments, they may fail to account for additional sources of electromagnetic interference in the real environments.
In view of these shortcomings, a need exists for improved systems for determining RFID tag placement. The conventional systems and related shortcomings are presented in this section for illustration purposes. However, the described shortcomings are not the only limitations of conventional systems. Other limitations of existing or prior systems will become apparent to those of skill in the art upon reading the following written description.